primary conductor size for transformer

[SceneryDriver]

I have a 45KVA 3-ph step-up transformer, 240D ---> 480Y, feeding a dedicated piece of equipment, hardwired. FLA of the 240V side of the transformer (primary) is 108A. Existing 150A breaker will be used to feed the transformer.

FLA of the machine is 51A @ 480V, or 102A at 240V. Do I size the feeder to the transformer for 102A (#2 THWN @ 75deg) or for 150A (1/O @ 75deg).

I do not plan to have secondary protection, as my primary OCPD is sized at 125% (135A), rounded up to the next standard size (150A). Thoughts?



SceneryDriver

 

[texie]

Size the 240 primary conductors to the 150 amp breaker. I would add that you need OCP on the 480Y side and a GES, etc.
Glad to see you are using the correct transformer.

 

[SceneryDriver]

Size the 240 primary conductors to the 150 amp breaker. I would add that you need OCP on the 480Y side and a GES, etc.
Glad to see you are using the correct transformer.

Thanks for the reminder. Of course a 3ph trafo needs OCP on the secondary. Not sure what I was thinking when I wrote that. Star point on the secondary side will be bonded to building steel / grounded.

Corner-grounded delta power is a mess. We have transformers EVERYwhere in the facility. Somehow that was cheaper than replacing the service when we moved into the building.


SceneryDriver